The present invention relates generally to processes for bonding contacts to contact bases, and more particularly to a process for bonding a contact to a contact base, in which the contact consists of a silver-metal oxide-(MeO)-based material, in which each individual contact, which has a surface on the contact mating area, side surfaces, and a back side for bonding to the support, is provided with solder on the back side. As used herein, contacts are to be understood as separate contact pieces or else tapes or shapes for the cutting off of individual contact pieces. The present invention also relates to a semifinished product which can be obtained by the process indicated.
U.S. Pat. No. 3,954,459 discloses process for manufacturing a sintered compound material for electric contacts, in particular for power engineering, in which a silver-metal oxide material is used. Cadmium oxide (CdO), stannic oxide (SnO.sub.2), zinc oxide (ZnO), lead monoxide (PbO) or ferriferrous oxide (Fe.sub.3 O.sub.4) or their mixtures are often considered for use as metal oxides in this application. In the case of these contact materials, in view of their poor wettability by liquid solder or their unfavorable welding properties, two-layer contact pieces having a layer of readily solderable or readily weldable metal are produced in order to obtain a reliable bonding technique. Single-layer contacts can possibly be imparted better soldering properties by dissolving the metal-oxide components on the solder side, such as by acid, for example.
Furthermore, U.S. Pat. No. 3,989,516 discloses a process for producing a material serving as an electrical contact, in particular on the basis of silver-cadmium oxide with possibly stannic oxide, in the case of which is the disturbing fact that in the prior art a non-homogeneous distribution of concentration with accumulations of cadmium oxide in the surface region of the contact pieces is formed. The specific problem of the bonding of the contacts to contact bases is not mentioned therein.
In the manufacture of switching devices, contact pieces are customarily fastened to the contact bases of the switching device by hard soldering. For this purpose, a film of solder is applied to the back side of the contact pieces or of the material which is present in the form of a longer tape, for example by roll-bonded cladding or the soldering-on of individual solder spots. Films of solder can also be bonded by ultrasonics and then pressed in place. The presoldered contact pieces are additionally processed substantially automatically and fastened to the contact base as part of an integrated manufacturing process, for instance by inductive heating.
In such energy-induced processes, problems can arise due to liquid solder ascending from the solder side over the narrow edges of the contact and coming onto the surface of the contact mating area. The switching behavior of the contact can be affected in an undesired manner by such uncontrollable processes upon use of the switching device in accordance with its intended purpose.
The present invention is directed to the problem of developing a process for the treatment of contacts by which the presence of parts of solder on the surface of the contact mating area of the contact after the hard soldering is prevented. The present invention is also directed to the problem of producing a semifinished product according to the process of the present invention.